Im afraid you can only get the cover as part of the light fitting. Its about £12 s if you get it you can just unscrew the cover of the new light fitting and pop it onto the old fitting inside the oven. They just don't sell the glass cover on its own.
Click here for the part
As to running it without the cover it should be fine for a short time. But ive see people who don't bother replacing the cover and just leave the bulb exposed. It wont cause any damage to the oven but I think it may cause the bulb to blow faster.
If you need anymore info just get back to me.